bivalve vs univalve

bivalve

noun
  • Any mollusc belonging to the taxonomic class Bivalvia, characterized by a shell consisting of two hinged sections, such as a scallop, clam, mussel or oyster. 

  • A pericarp in which the seed case opens or splits into two parts or valves. 

univalve

adj
  • having one valve; typically used to refer to mollusks, notably slugs and snails. 

  • consisting of a single valve or piece, used to refer to a shell. 

noun
  • A univalve mollusk or its shell. 

How often have the words bivalve and univalve occurred in a corpus of books? (source: Google Ngram Viewer )